SOLUTION: Christie's mother gave her $9.00 to buy 10 cent and 15 cent stamps. Christie returned with $1.75 in change and a total of 60 stamps. How many of each kind of stamp did she buy?

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Question 451516: Christie's mother gave her $9.00 to buy 10 cent and 15 cent stamps. Christie returned with $1.75 in change and a total of 60 stamps. How many of each kind of stamp did she buy?
Found 2 solutions by rwm, pedjajov:
Answer by rwm(914) (Show Source):
You can put this solution on YOUR website!
t+f=60
10t+15f=900-175

Answer by pedjajov(51) (Show Source):
You can put this solution on YOUR website!
Let's use the following:
t - number of 10 cents stamps
f - number of 15 cents stamps
Everything has to be expressed in cents.
Money Christie used is -> $9 = 900 cents
Total number of stamps is 60 -> t + f = 60
Total value of stamps is -> 10*t + 15*f
Christie returned $1.75 -> $1.75 = 175 cents
Value of the stamps is -> 10*t + 15*f = 900 - 175 = 725
We have now two equations with two variables (unknowns):
t + f = 60
10*t + 15*f = 725
If we solve first equation for t, we get -> t = 60 - f
Now replace t in the second with the expressed t from the first equation
10*(60-f) + 15*f = 725
600 - 10*f + 15*f = 725
600 + 5*f = 725
5*f = 125
f = 25
t = 60 - 25
t = 35
check: 10*35 + 15 * 25 = 350 + 375 = 725, OK